Graphical

Tk:

var tk = require('Tk')
var main = %O<MainWindow>.new
main.Button(
    '-text'    => 'Goodbye, World!',
    '-command' => 'exit',
).pack
tk.MainLoop

Gtk2:

var gtk2 = require('Gtk2') -> init
 
var window = %O<Gtk2::Window>.new
var label  = %O<Gtk2::Label>.new('Goodbye, World!')
 
window.set_title('Goodbye, World!')
window.signal_connect(destroy => { gtk2.main_quit })
 
window.add(label)
window.show_all
 
gtk2.main

Gtk3:

use('Gtk3 -init')

var gtk3   = %O'Gtk3'
var window = %O'Gtk3::Window'.new
var label  = %O'Gtk3::Label'.new('Goodbye, World!')

window.set_title('Goodbye, World!')
window.signal_connect(destroy => { gtk3.main_quit })

window.add(label)
window.show_all

gtk3.main